The present invention relates to a method of rendering or reproducing the half tone of a colored original by digitally producing a screen dot image having any screen angle and any screen pitch on the basis of image data read from the colored original by an image input device such as a color scanner.
In the case where a print is produced from image data with gray levels read from a colored original by an image input device such as a color scanner, the colored original is photoelectrically scanned to obtain three primary color signals. Next, the color signals are subjected to color correction and are thereafter converted into image signals for color separations "magenta", "cyan", "yellow" and "black" Further, there is generated a screen dot which has a screen percentage corresponding to a gray level of the image signal. The screen dots are periodically arranged and the period of arrangement is called a screen pitch. A photosensitive recording medium is exposed to light from a light source luminance-modulated by a recording signal for generating the screen dot so that each color separation is obtained. Thereafter, superimposed or registered printing is made by a printer using inks of colors corresponding to the color separations, thereby obtaining a final image rendered.
In actuality, printing with accurate registration of screen dots of the respective color separations is not possible and a minute deviation in screen dot period and screen dot direction between the color separations yields moire pattern. A fringe pattern or the like, which is not included in the original, appears in a print due to the moire pattern, thereby deteriorating the renderability or reproducibility of the original. It is possible to make the moire inconspicuous by changing an angle at which screen dots in each of the four color separations are arranged. This angle is called a screen angle. It is known that the combination of screen angles of 0.degree., 15.degree., 45.degree. and 75.degree. is preferable in order to minimize an influence of the moire in a four-color printing.
The conventional method of recording an image of screen dots having such a screen angle includes a method in which a lith type film for recording is brought into close contact with a contact screen and is subjected to exposure.
In recent years, on the other hand, various methods of electronically forming screen dots have been developed and put into practical use. The fundamentals of those methods are such that a recording medium is divided into recording elements smaller than a screen pitch and a digital recording signal for determining whether or not the recording element is to be recorded is generated in accordance with the gray level of an image signal so that the recording medium is subjected to exposure. In general, a matrix of threshold values is used in generating the recording signal. Namely, a matrix element corresponding to the recording element is selected, and a threshold value of the selected matrix element and a gray level of the image signal are compared to generate a recording signal in accordance with a relationship in magnitude between both the values. Screen dots can be generated periodically by using the matrix elements periodically. In this case, what is important in order to prevent the moire pattern from occurring, is how to generate screen dots having a desired screen angle.
One example of a method of electronically generating screen dots having a desired screen angle includes a screen dot image generating method disclosed by U.S. Pat. No. 4,499,489. However, a screen dot image generated by this method involves a problem that the number of pixels forming each screen dot and the form of a screen dot are different even in the case where a fixed gray level is represented and an undesirable ruggedness may appear in the form of the screen dot. Especially, in a region where the form of a screen dot is small, there is a problem that a reproduced image exhibits an undesirable appearance rough to the feel, thereby resulting in the deterioration of quality of the reproduced image.